1. Field of the Invention
The present invention relates to an ultrasonic machining method for mirror-finishing (polishing) a hard workpiece made of boride cermet which has both a hard layer and a comparatively soft metal binding layer.
2. Description of the Prior Art
Ultrasonic machining for polishing is usually accomplished by the impact action of hard abrasive grains placed between a workpiece and a tool attached to a vibrating horn of an ultrasonic machine. The impact action is induced by the ultrasonic vibration of the tool. The abrasive grains, which are made of meta-B.sub.4 C or CBN, bring about minute brittle fracture in the work surface of the workpiece. In other words, ultrasonic machining resorts on the mechanical removal of material from a workpiece which is achieved by the direct application of mechanical force (machining stress) to a workpiece. Therefore, the machining precision of the finished surface is no better than that achieved in mechanical machining.
In order to improve the accuracy of finishing, there was invented and proposed an ultrasonic machining method which is combined with electrochemical machining. According to this method, an electrolyte containing hard abrasive grains is placed between a workpiece and a tool and an electric current is applied across them for electro-chemical machining. (See Japanese Patent Laid-open No. 282821/1987.)
This new ultrasonic machining combined with electrochemical machining provides a higher accuracy of finishing than the conventional ultrasonic machining Nevertheless, it still has a disadvantage that it leaves strain in the workpiece because it achieves machining by the impact of hard abrasive grains against the workpiece or by the mechanical force directly applied to the workpiece. Therefore, it is not yet satisfactory for mirror finishing which needs an extremely high precision.
There has recently arisen a demand in the field of die cast mold for the technology which permits mirror finishing with an extremely high precision. The cavity of a die cast mold into which a high-temperature molten metal is injected should have a mirror-finished surface so that it gives a good casting surface and a high dimensional accuracy to the molded product.